codicil vs appendix

codicil and appendix both are nouns.

Word	Noun	Adjective	Verb	Adverb
codicil	Yes	No	No	No
appendix	Yes	No	No	No

As nouns, appendix is a hypernym of codicil; that is, appendix is a word with a broader meaning than codicil:

codicil: a supplement to a will; a testamentary instrument intended to alter an already executed will
appendix: supplementary material that is collected and appended at the back of a book

codicil (noun)	appendix (noun)
a supplement to a will; a testamentary instrument intended to alter an already executed will	a vestigial process that extends from the lower end of the cecum and that resembles a small pouch
	supplementary material that is collected and appended at the back of a book